For more than forty years, the problem of acceleration sensitivity of quartz crystal resonators has been with us. Even in the 1940's "shock and vibration" was one of the biggest problems awaiting solution. A variety of approaches have been used: mechanical isolation, using springs and damping elements to reduce the level of distrubance; electrical compensation, using accelerometers to sense the forces and provide input to correction networks; determination of mounting locations on the crystal for the supports to minimize the effect; different crystal cuts; different types of vibrations; different geometrical shapes and aspect ratios; and use of two or more crystal resonators connected electrically, but having reversed axial direction with respect to the acceleration vector. These schemes have met with mixed success.
The average acceleration sensitivity for AT-cut BAW resonators is .gamma..congruent.10.sup.-9 per g or earth gravity unit. Turning a crystal resonator 180.degree. with respect to "down" results in a change of two earth gravity units, so the "2-g tipover" test usually yields a fractional frequency change of ##EQU1## In many areas of metrology, a change of this size would be considered negligible; but frequency can easily be measured to parts in 10.sup.13, and modern communication and sensing systems require stabilities of such a level that 10.sup.-9 frequency changes are not allowable.
The acceleration level in a helicopter can be EQU a=3 to 6 g's
which then introduces a frequency modulation (FM) at the variable rotor blade frequency, rendering certain types of equipment inoperable, and degrading the operation of others. Remotely piloted vehicles (RPV's or UAV's) are similarly affected, as are tanks. In the civilian, commercial sector, cellular radio communications are degraded, and certain modulation schemes cannot be used, or are degraded by the vibration of the using vehicles. Many other examples could be cited; the acceleration problem is the most serious obstacle to be overcome for most applications of crystal resonators (both BAW and SAW) in the HF, VHF, and UHF bands; the same problem exists for dielectric resonator oscillators (DRO's) and magnetostatic wave (MSW) devices at higher frequencies also.